Patent GB-961710-A (GB)
Title: Improvements in or relating to junction transistors
IPC: H01L23/482
Filed: 1960-08-02
Abstract: 961,710. Transistors. TELEFUNKEN A.G. Aug. 2, 1960 [Aug. 6, 1959], No. 26728/60. Heading H1K. A junction transistor comprising emitter and collector zones on opposite sides respectively of a semi-conductor wafer, has only two connections to the emitter side of the wafer, the first in ohmic contact with the base zone and the other in ohmic contact with the base zone but also in contact with the emitter zone ohmically or through a junction with a low breakdown voltage. The two contacts to the base zone enable emission to be restricted to a small part of the emitter junction as in the normal two base transistor. The arrangement however has the advantage that the two contacts and associated leads may be more massive and therefore of lower resistance and inductance than when three contacts are used. The embodiment shown in Fig. 4 in which a substantially intrinsic zone 8, the thickness of which is a minimum beneath the emitting part 13 of the emitter zone 3, is interposed between base 4 and collector 5 zones is made as follows. Indium is alloyed to one face of a thick slice of nearly intrinsic N germanium to give a thick monocrystalline P-type layer. After alloying a contact to the edge of the N zone and masking the P zone with insulating varnish the junction is reverse biased and the N zone etched electrolytically to the edge of the depletion layer only where the minimum zone thickness is required by illuminating only that part with a scanning strip of light. The reverse bias is subsequently increased and the etching repeated but with the entire surface scanned by the light. After removal of the varnish and indium layer a vacuum deposited layer of indium-gallium alloy containing a small amount of antimony or arsenic is alloyed to the intrinsic zone to form a P zone from which the arsenic or antimony is diffused in a subsequent heat treatment to form the N-type base zone. After removal of the residual alloy in acid and subdivision into dice 80 Á diameter pellets 1, 2 of indium-antimony-silver alloy are alloyed through the treated surface of each slice to form N zones 6, 7 through which ohmic contact is made to the base zone. A strip of photoresist 70 Á wide is applied along the line joining the pellets and the surface etched to mesa form. On removal of the photoresist zinc is deposited over the upper surface and then etched away through a photoresist mask to leave only layer 14 in ohmic contact with the emitter zone. Collector zone 5 is finally soldered to base 10 and silver leads 11, 12 soldered to pellets 1, 2. In forming an otherwise similar device with a uniformly thick intrinsic zone the first electrolytic etching step in the above process is omitted. Where no intrinsic zone is required the indium-gallium alloy containing arsenic or antimony is deposited on a .3-1 ohm cm. P-type wafer. Subsequent manufacturing steps are the same. A further embodiment, Fig. 5, with circular and annular base contacts 1, 2 suitable for connection into a coaxial line is formed by first alloying acceptor material into an annular region of one face of a weakly N wafer to a depth of 10-15 Á, applying a thin indium layer to the entire face, superposing a second wafer and then alloying with the second wafer at the higher temperature to give the resultant P zone configuration 5. The junction is reverse biased and the N surface electrolytically etched as before until the etching reaches the depletion layer in annular region 13 which becomes visible as a bump. Subsequent treatment is similar to that described above. In alternative methods in which the base and emitter zones are formed by successive diffusion steps the regions 6, 7 are masked during formation of the emitter zone and contacts made to them after the masking is removed. The base zone is heavily doped and the emitter even more heavily to ensure a high emitter efficiency. If zone 7 is also heavily doped to give a low Zener breakdown voltage junction to the emitter the junction may be used to connect contact 2 to the emitter and layer 14 dispensed with.